Surface and near surface regions of materials exposed to high temperature environments, whether intentionally, such as in a furnace, or as a result from ordinary service, such as components used in hot gas paths of gas turbine engines, undergo changes in material properties. For instance, case hardening occurs when steel is exposed to ammonia gas resulting in nitride formation, which causes enhanced component wear performance. In addition, bright annealing of the surface may occur through the elimination of surface oxides. Embrittlement of the general microstructure near the surface may occur from nitride formation, and weldability and fatigue performance may be reduced. In addition, the grain boundaries may be weakened from excessive oxide formation, which may reduce weldability and fatigue strength.
A significant reduction in component life can result from such negative changes because the ability to repair the part is greatly reduced. One such example is a gas turbine component downstream of a combustor. The internal surfaces of transitions in a gas turbine engine are often coated with a thermal barrier coating to protect the material from reaction with combustion gases and to limit metal temperatures. The outer surfaces are typically not coated with the thermal barrier coating and undergo reaction with the atmosphere at elevated temperatures. After long term usage in a turbine engine, precipitation of oxides and nitrides, such as aluminum nitrides (AlN), occurs near the surface. Such precipitations cause embrittlement, which, when weld repairs are attempted, often results in cracking.